Radio frequency (RF) circuitry is implemented using differential signaling in order to eliminate common mode noise and increase dynamic range. However, real life signals like those received by an antenna or transmitted by an antenna are single-ended. So, somewhere in the receiver (RX) and transmitter (TX) circuitry, the signal is converted from single-ended to differential or from differential to single-ended. A device that utilizes magnetically coupled primary and secondary coil windings to convert radio frequency (RF) signals that are balanced about a common mode (viz., ground) reference (i.e., differential signals) to signals that are unbalanced (i.e., single-ended signals), and vice versa, is called a “balun.” This conversion is typically done right after the low noise amplifier (LNA) stage for the RX chain and right after the power amplifier driver (PA Driver) stage for the TX chain. It can also be done right before the LNA and right before the PA Driver, respectively.
In addition to providing signal conversion, baluns are also used to provide impedance transformation between preceding and following stages. This allows for maximum power and signal transfer. Impedance matching is accomplished by changing the turns ratio between the two sides (primary and secondary) of the balun. This provides the ability to either step up or step down the impedance. Further, the physical value of the balun inductance may be an important parameter in enabling resonance to be achieved at the operating frequency. Especially in low voltage, high current applications, the DC resistance of the primary side may be important because it translates into lost headroom, thereby impacting linearity.
It is generally desired to keep the loss of baluns to a minimum. However in many digitally-oriented advanced deep submicron processes, back end-of-line (BEOL) processing steps typically use very thin metal layers in order to maximize the efficiency of routing for digital applications. And, with continuing downscaling of process dimensions, the thickness of metals used in BEOL layer stacks continues to decrease. Such thin metals, however, have very high RF loss (poor quality factor Q). In many situations, only one thick copper (Cu) metal layer may be made available for power routing. A thick aluminum (Al) redistribution layer (RDL) is also usually available for bonding (for wirebond or stud-bump flip-chip packaging). Copper is generally not currently used for bonding because of its high malleability. Aluminum, however, has higher resistivity and very poor electromigration characteristics.
In general, higher frequency baluns, such those implemented for wireless local area network (WLAN) applications at 2.5 GHz and 5.8 GHz frequencies, may be easier to realize on chip because of smaller area (need for small inductance) and higher Q with higher frequency. Low frequency cellular band baluns are generally more difficult to realize and are typically implemented in a single in-line package (SIP) configuration in an integrated product development (IPD) process or on a printed circuit board (PCB). See, J. Mondal, et al., “Design and Characterization of an Integrated Passive Balun for Quad Band GSM Applications,” IEEE Electronic Components and Technology Conference, pp. 534-540, 2006, incorporated herein by reference.
The conventional approach to integrating baluns together with other components on a same chip has been to use a stacked arrangement of primary and secondary coil windings of two superposed differential inductors, with one coil formed using a first thick copper metal layer and the other coil formed using a second thick copper metal layer. At least two additional metals are needed to implement the cross-unders/crossovers for the two differential coils, respectively. The accomplishment of such approach in an integrated circuit fabrication flow, such as a modern CMOS process flow, that otherwise requires only a single copper layer is, however, disruptive and expensive because additional metal layer and masking steps have to be added that would not otherwise be necessary.
To avoid the need for adding another copper layer and masking step into a chip fabrication process with only one thick copper metal layer available, on-chip baluns have been implemented using coplanar configurations wherein the windings of primary and secondary coils of two differential inductors run side-by-side and magnetically couple in a coplanar fashion. In the coplanar implementation, the single thick copper layer is used for the side-by-side placement of the coil windings, with the overlying thick aluminum and underlying thinner copper layers being used for the crossover and the cross-under connections of the interwoven windings, respectively. The magnetic coupling of coplanar coil windings is, however, inferior to that of stacked windings and, because the coplanar coupling factor (k) becomes severely degraded for higher turns ratios. Further, for coplanar arrangements, a higher turns ratio implies larger area and longer crossovers. This can degrade the loss of the balun further since the longer crossovers are implemented typically using the poorer conductivity aluminum. Because of the coplanar coupling, a 2:1 turn ratio is typically the highest that is implemented.